I have a question please clarify, why you used "i" - Interest value as is which is 12%, while in problem it says, nominal interest rate of 12% compounded monthly. Does this mean that, we have to determine the monthly effective interest, which will equal to 12/12=1% !!!
Why I am asking, because, I need to submit the solution, and show the approach i used, I mean I must use the "formula" , when I am using formula, I was considering monthly effective interest as 1% in the "Increasing Annuity-Immediate Present Value"..
If I want to use your figures, which formula I will have to use, if we want to solve it other than excel?
The way I solve such questions is to assume a random value for payment P to begin with, find the sum of PV of all payments and then use Goal seek tool of excel to find exact value of P.
If we look at formula based approach then we have to go for 'Annuities whose payments follow an arithmetic progression'. There is a paper which explains this technique, if you want I will share the link with you, but it is not my work.